One day in your average japanese library, 21 year old college student Naofumi Iwatani found an intriguing fantasy book in the light novel section called "The Four Cardinal Heroes" that talked about legendary heroes with each possessing a powerful weapon such as the Spear, Sword, Bow or Shield. He is then suddenly sucked into the book after flipping a blank page and gets summoned to another world with three other teens to save mankind from The Waves of Calamity, a series of demonic invasions from another dimension. After meeting the king of the country of Melromarc where the four were summoned in, Naofumi discovers he is the legendary Shield Hero and later at night he learns from the other Heroes who were each summoned from different versions of Earth, that the Shield Hero is considered the "loser class" for being the weakest of them all.

When the next day came and the party members were assigned, nobody joined Naofumi initially, until a member of the Spear Hero's party by the name of Myne Sophia offered to join him. But while he is initially excited at going on a grand adventure, Myne steals his belongings and accuses him of raping her in the very next day. Naofumi's denials of the crime and pleading of his Innocence were ignored by all, with him being labeled a criminal and becoming an outcast, he was only shown kindness by Erhard, the weapon-smith he met in his first day in the city who realizes the hero is innocent. Embittered and enraged, Naofumi leaves the city walls to begin scrounging for materials to use or sell and he learns how to upgrade his shield with any resources he finds in this world. After a few days he gets caught up fighting away some thugs who wanted to rob him, Naofumi realizes he can't upgrade without skill or strength. But then he is approached by a slave trader named Beloukas and Naofumi is brought to a slave market so a slave may gain XP by fighting for him.

While Beloukas offers Naofumi a high-level beast man wolf, in the corner of the room however Naofumi finds a demi-human slave girl cowering in a cage. Naofumi buys the much cheaper Tanuki girl, Raphtalia. They place a slave crest on her to prevent Raphtalia from betraying or disobeying Naofumi's orders without feeling intense pain, while also linking shared experience gained by one to the other. Raphtalia's recurring nightmares of her parents'deaths in the first Wave have deeply scarred her, making her initially hesitant to kill but Naofumi uses the crest to force her into fighting. Both leave the capital for the small town of Lute to exploit a mine to collect valuable ore, but they learn that a monster has occupied the place and is killing anyone who enters with the shield hero seeing this as an opportunity, offering to kill the monster for a good amount of money. Once inside the mine, they're attacked by a high-level dog monster with two heads that is able to wound Naofumi, this being the same beast type which killed her parents in the Wave, the monster's appearance sends Raphtalia into a paralyzed shock. Seeing her fear, Naofumi feels empathy for the first time ever since the trial and tells her to escape while he buys time, putting himself in danger for the sake of someone else. His words make the young tanuki girl remember her parents sacrifice to save her life, gathering the strength to confront her feelings of regret and helplessness, Raphtalia summons up the courage to kill the beast. Naofumi comforts Raphtalia afterwards, promising not to leave her and she agrees to help him defeat the Waves so other children won't suffer as she has.

Later at night time, Naofumi and Raphtalia are resting on a room in the inn which the village lord allowed them to stay for free since they defeated the monster when no one else did and while raphtalia prepared her bed, Naofumi was checking his menus from the status magic. Seeing what shields he should master next, the first one would obviously be the [two headed dog shield] as its ability [Alertness] would be the most useful one for when the wave hit, being alerted when a enemy is close by in a 30 meters radius would make surprised attacks mostly non existent as he would still have to rely on his reflexes to block those attacks but he would know they were coming. He also checked his drop menu to see what the black dog had dropped, he located a amulet listed as [Double Spirit Dog Amulet] which gave him 10 Sp and 5 speed when equipped, but the intriguing part was that it had a ability but he couldn't see what it did, so he took the amulet out of the drop menu and tried to analyze it. It was a jasper amulet with a hexagonal rock with a drawing of two dogs chasing each other, the white and black color scheme of each dog made the form of a ying yang symbol, he could only see its rarity now which was quite high as this was listed as a interdimesional rarity. He could wear it around his arm or give it to raphtalia but considering how much she hated that dog, it was best for him to keep it for himself

"This has a unique ability, but I can't see what it is... might as well absorb it"

[Double Spirit Dog Amulet Shield unlocked!]

[Equip Bonus 10 SP and 5 Speed]

[Unique Ability Double Soul Split : Processing...]

"Ugh!" Naofumi grunted in pain, placing a hand to his chest while trying to not fall on to the ground, this alerted raphtalia who rushed to his side upon seeing the shield hero shaking in pain, briefly noticing his new shield

"Master Naofumi! What's wrong?" She desperately asked, she didn't know why this was happening but she could assume that some sort of spell was cast on him by this new shield or that his wounds in the fight against the two headed dog hadn't healed correctly despite the great quality of Naofumi's healing potions

"My chest ... It's like my insides are spinning and going into opposite sides... I feel like I'm being ripped in half..."

"What the hell... is this shield doing!? I can't stop it!"

"Argh!" the shield hero screamed out as his entire body became pure light that blinded raphtalia for a moment before she saw Naofumi was now laying down against the wall of the room unconscious, the wall behind him seemed to have been hit with great force as the wall paper and part of the wood had been damaged

"Naofumi!" the raccoon girl rushed to his side once more, trying to see if he was well or if he wasn't severely injured

"Are you alright!? Please say something!" she screamed out to him while shaking his body to see if he would wake up, she didn't want him to die, after all the passed through and what they did in the mine... She cared about his safety now and she didn't want for his life to end so shortly before he could show to the world his

"Shut... Shut the fuck up! I'm fine..."

"What was that?"

"I don't know, it seems to just make most of my clothes black"

"What happened to the white dog carving?"

"Raphtalia are you alright? My head is spinning like crazy..." the voice of Naofumi called out but... from the other side of the room. Raphtalia and the Naofumi by her side looked to see another Naofumi recovering from his own crash into the wall, he was wearing the same outfit as the other Naofumi only it had white instead of black. When this Naofumi in white saw his counterpart with raphtalia by his side, he visibly raised his eyebrow in confusion in a almost comedic way

"..." the three stayed in silence while trying to understand the situation at hand, confusion was filling the entire room

"What... The... Fuck!?" Both Naofumi's say in union while the two get up, the Naofumi in white has a shock expression that quickly turns into a terrified face while the Naofumi in black has a expression of surprised anger on his face the whole time

"Who the hell are you!? Raphtalia quick, kill him!"

"B-but he is-"

"Kill me?! What's your deal? Raphtalia you better get behind me, this faker could be dangerous"

"Faker? I think you're the only fake Naofumi here"

"Let's calm down and figure this out! I have no idea what's happening"

"What caused this?" the Naofumi in white asked himself and a notification in the status magic appeared to him and the Naofumi in black

[Unique ability of the Double Spirit Dog Amulet Shield in effect]

[Unique Ability Double Soul Split: when this shield is equipped, your body and soul are divided into two as your positive traits and negative traits are put in different bodies. This ability will stay active even if the wielder changes the shield form, it can only be deactivated once both halves agree to join back]

"What... So this is the shield's doing?"

"I... I am going to assume you're my bad side?"

"pfft, bad side? I am the normal one you little piece of shit, looking at you is like seeing a pathetic version of myself in a mirror, bet you're a little bitch and-"

"Naofumi-sama"

"... okay, perhaps I am the bad side, though I prefer "negative" over bad one"

"This is surreal... two Naofumi-samas, I am at a loss for words"

"Yeah me too"

"It says that if we both agree to join back, the [double spirit dog shield] will deactivate its ability"

"Great! what are you waiting for, do it"

"Pass, I don't feel any different from before and what guarantees that fusing back would even work? I'm fine as I am and there is no way I'm letting my guard down in front of that half baked version of me"

"What!?"

"Yeah ... I don't want to go back to being my normal self either, my shoulders feel lighter and there isn't a single negative thought going on in my head right now. I could probably sleep without having to worry"

"That's a weakling's mindset, just another reason for me to not join back into an inferior version of myself"

"But-"

"Shut up, you're my slave so you don't have a saying in this"

"Hey! you shouldn't talk to her like that, she might be yo- our slave but she is also our party member"

"I don't need a sermon from the mister 'white knight' of all people"

"Well I just think that as a Hero, we should try to be better"

"If you think a single person in this shit hole of a country cares about us then you're delusional, Raphtalia! we are going out to grind"

"What about the other... master Naofumi"

"He can die in a trench for all I care"

"Thanks for the sentimentality"

"No I mean what are we going to do about having two of you?"

"..." the two half's stay silent for a moment as they think about the situation

"Damn... she's right, if someone sees two versions of me then it could put us in a tuff spot"

"I bet that lying snake tongue bitch and her demented king of a father would use this against me..."

"Then... you just need to become one again"

"No!" both Shield heroes shouted, the two had already made clear why they didn't want to fuse back and they wouldn't change their minds so easily just because a predicament appeared in the way, they just had to figure out a way around it

"We'll just have to not be seen together, that's all"

"I got a plan, you go South and I go North. That way I don't have to see your stupid face"

"We have the same face"

"Whatever, me and Raphtalia are going to grind out some monsters and you... can fix this room before anyone gets in, you made quite a mess on the wall"

"We made this mess! but you're right, it is a polite and good thing to do"

"Great, once you're done you can fuck off far away from my sight"

'... was I always this rude?' the Naofumi in white asked himself in his head, he remembered the awful and admittedly criminal things he did before being split but his outlook of them was different. Now he just saw them as petty and uncalled for, Light Naofumi could only reason that his previous self saw those actions as justified because of how he was treated like a criminal but that wasn't the case anymore, now he just couldn't call those memories as his since that was a version of him full of malice and negativity that he didn't had anymore, or rather his counterpart had.

things were back to normal... or at least close to what Raphtalia was used to by this point, she and this dark Naofumi were fighting off monsters close by the Lute village forest. She was having a much easier time fighting and more importantly killing the monsters, she didn't flinch when she saw blood now nor did she hesitate when attacking, after killing the two headed dog in the mine it was like she had killed her own incapability and her fears. She could also see the difference between this Naofumi and the one she knew, this dark Naofumi was much more aggressive in how he was battling the monsters and the way he used the [two headed black dog shield] to viciously counter their attacks also felt more brutal than what her master would do, this one keep reminding Raphtalia of Naofumi since this one was by far the most similar of the two to the original Naofumi in how he acted or talked, but it was more aggressive and he didn't seemed to worry about if Raphtalia got hurt, he just wanted her to fight for him and for her to not complain against his orders

'This is like how it was before master split himself... but it feels so different...' The young raccoon girl thought to herself, hoping that the fight would get her head out of this feeling but it wasn't doing much.